In making semiconductor devices, particularly integrated circuits, the semiconductor substrate is coated with a layer of silicon oxide and openings are provided through the silicon oxide layer through which extends a conductive layer to make contact with the substrate. If the edge of the opening is sharp, breaks or thin sections can be formed in the conductive layer where it passes over the sharp edge. Such breaks or thin sections are undesirable in that they provide electrical openings or areas of increased resistance in the conductive layer. Therefore, it is desirable to taper the sides of the openings to eliminate the sharp edges. One technique which has been found suitable for forming tapered openings is described in U.S. Pat. No. 3,839,111 to E. J. Ham et al., issued Oct. 1, 1974, entitled "Method of Etching Silicon Oxide To Produce A Tapered Edge Thereon". In the process described in U.S. Pat. No. 3,839,111, the silicon oxide layer is coated with a layer of a resist which is provided with an opening therethrough. As the exposed portion of the silicon oxide is etched away, the etchant causes the resist layer to lift up from the surface of the silicon oxide at the edge of the opening resulting in a tapered wall of the opening.
A problem which has been found in using this process on a production basis is that the angle of the taper varies with the season of the year. I have found that this seasonal variation is a result of variations in humidity which causes variations in the water content of the silicon oxide layer. The water content of the silicon oxide layer affects the adhesion of the resist layer to the silicon oxide. This, in turn, affects the lift off capability of the resist layer which affects the taper of the opening etched in the silicon oxide layer. For mass production of semiconductor devices it is desirable to have a process which produces openings in the silicon oxide which are of uniform taper.